Quite a power
house in a small package
A universal motor is a motor capable of running on either AC or DC current (that's what's "universal" about it). It consists of an armature with coils wound on it suspended within a cage of field coils. The coils on the armature are connected to a commutator, which allows the magnetic field produced to shift as the armature turns. This is very similar to the operation of a permanent magnet DC motor.
Normally if you connect a DC motor to alternating current, the drive shaft will simply vibrate at whatever the frequency of the line is. This is because the direction of a DC motor is dependant on polarity, and AC polarity reverses many times per second.
A universal motor uses field coils instead of permanent magnets, however, and these coils are powered by the same current. This causes the magnetic field around the armature to reverse in unison with the field produced by the armature, causing rotation in one direction.
Universal motors are extremely powerful for their size and weight, compared to induction motors. They tend to run at extremely high RPM's (15,000 to 20,000 is not uncommon) and have very good starting torque, which makes them suitable for applications such as power drills, electric weed whackers, and blenders. The downside is that, due to commutation, universal motors are very loud and have nowhere near the operating life that an induction motor has. The commutator also produces sparks which make these motors unsafe for use near flammable materials.